In order to implement overcurrent protection for constant-voltage power supply circuits employing series regulators, overcurrent protection circuits have been widely used such as, for example, a current limiting circuit configured to prevent a current output from exceeding a predetermined current, and an overcurrent protection circuit configured to limit a current output under short circuit conditions.
The overcurrent protection circuit is characterized by the so-called fold back current limiting capability, which provides better protection than conventional current limiting because as a load resistance decreases below a predetermined value, both the voltage and current decrease simultaneously along a characteristic foldback locus.
FIG. 3 is a diagrammatic circuit diagram illustrating a known constant-voltage power supply circuit incorporating an overcurrent protection circuit.
Referring to FIG. 3, the known constant-voltage power supply circuit 100 includes a constant-voltage circuit 101 and an over-current protection circuit 102.
The constant-voltage circuit 101 includes a reference voltage generating circuit 111 configured to generate and output a predetermined reference voltage Vref, an error amplification circuit AMP, an output transistor M101, and resistors R101 and R102, configured to generate and output a partial voltage VFB, which is obtained by dividing an output voltage Vo.
In addition, the over-current protection circuit 102 includes PMOS transistors M102, M103, M106, and M107; depletion-type NMOS transistors M104 and M105; a resistor R103, and a bias current source 112.
In the case when a current output from the output transistor M101 is smaller than a predetermined current value for exerting over-current protection, the drain current of the current detection transistor M2 is relatively small and the voltage drop by the resistor R103 becomes smaller than the sum of the partial voltage VFB at the junction between the resistors R101 and R102, and an offset voltage Vof. This makes the depletion-type NMOS transistor M105 turned off.
As a result, the gate voltage of the depletion-type NMOS transistor M105 is brought approximately to the input voltage Vin, the PMOS transistor M103 is turned off, and no over-current protection is performed.
When the current output from the output transistor M101 reaches the predetermined current value for exerting over-current protection, the voltage drop by the resistor R3 is brought to be equal to the sum of the partial voltage VFB and the offset voltage Vof.
As a result, the depletion-type NMOS transistor M105 is turned on, its drain voltage is decreased, and the PMOS transistor M103 is turned on.
By turning the PMOS transistor M103 on, the gate voltage of the output transistor M101 is elevated, the increase in the current output from the output transistor M101 is suppressed, and the output voltage Vo decreases.
Since the gate voltage of the depletion-type NMOS transistor M104 decreases with the decrease in the output voltage Vo, the over-current protection can be made operative for a small voltage drop by the resistor R3, and the output current io decreases with the decrease in the output voltage Vo.
When the output terminal OUT is short-circuited to the ground potential, the gate voltage of the depletion-type NMOS transistor M105 becomes equal to offset the voltage Vof.
During the short-circuit of the output terminal OUT, the magnitude of the current output from the output transistor M101 as the short-circuit current, is equal to the product of the current through the resistor R3 multiplied by the ratio of current between the output transistor M101 and the current detection transistor M102. That is, the magnitude of short-circuit current can be set by the values of the offset voltage Vof and the resistance of resistor R3.
In the abovementioned over-current protection circuit 102, however, the depletion-type NMOS transistor is used as an input component of the error amplification circuit in order to make the error amplification circuit operative at the voltages as small as approximately 0 (zero) V.
Since the depletion-type NMOS transistor has a gate voltage smaller than the source voltage in the range of small drain current, a certain degree of source voltage is required for the depletion-type NMOS transistor M104 or M105 even after the gate voltage of depletion-type NMOS transistor M104 is decreased to 0 V at short-circuit conditions.
Therefore, a difficulty with this over-current protection circuit is that the drain voltage of the depletion-type NMOS transistor M105 cannot be decreased to sufficiently low.
Along with the recent efforts to reduce power consumption in various equipments, circuit voltages also have been decreasing as typically evidenced by compact, portable electronic devices.
For example, the voltages input to the series regulated constant-voltage power circuits have been supplied recently with a minimum necessary voltage after processed once with a step-down DC-DC converter.
Moreover, the input voltage itself has shifted to as low as approximately 1.5 V, and problems are caused such as, for example, the gate voltage of PMOS transistor M103 cannot decrease low enough and this in turn brings the over-current protection circuit non-operative as long as the depletion-type NMOS transistor is used as the input component of the error amplification circuit, as mentioned earlier.
In order to address this problem and provide an over-current protection circuit capable of operating at low voltages, another known circuit is disclosed as illustrated in FIG. 4 (Japanese Laid-Open Patent Application No. 2004-118411, for example).
The components in the over-current protection circuit of FIG. 4 that are similar to those of the constant-voltage power supply circuit described earlier in reference to FIG. 3 are shown with identical numerical representations.
Referring to FIG. 4, the over-current protection circuit 100 includes PMOS transistors M112, M113, M114, and M115; an NMOS transistor M118; and resistors R113 and R114.
The PMOS transistors M112 serves as a current detection transistor, which is configured to output a current proportional to the output current from an output transistor M101.
The over-current protection circuit of FIG. 4 has a device structure similar to the constant-voltage power supply circuit of FIG. 3, with the exception that PMOS transistors M114 and M115 are included in the input circuit of error amplification circuit, and that the drain current of the detection transistor M112 serves as a bias current of the error amplification circuit.
The drain current of the detection transistor M112 is then distributed to PMOS transistors M114 and M115, and converted into a voltage with the resistor R113.
In the case when a current output from the output transistor M101 is smaller than a predetermined current value for exerting over-current protection, the drain current of the detection transistor M112 passes while evenly shared through PMOS transistors M114 and M115. Since the drain current is small, the voltage drop by the resistor R113 is also small.
As a result, the PMOS transistor M113 is turned off and no over-current protection is performed.
When the current output from the output transistor M101 reaches the predetermined current value for exerting over-current protection, the voltage drop by the resistor R113 reaches the threshold voltage of NMOS transistor M118 and the transistor is turned on.
By turning the NMOS transistor M118 on, the gate voltage of the PMOS transistor M113 decreases to be turned on, the gate voltage of the output transistor M101 is elevated. As a result, the increase in the current output from the output transistor M101 is suppressed and the output voltage Vo decreases.
Since the gate voltage of PMOS transistor M114 decreases along with the decrease in the output voltage Vo, the current through the PMOS transistor M114 and accordingly through the resistor R113 increases. The output current from the output transistor M101 decreases therefore with the decrease in the output voltage Vo.
However, a difficulty with the circuit configuration of FIG. 4 is that the output current from the output transistor M101 is one half of the output current maximum under short-circuit conditions, and it is not feasible for the value of output current be set arbitrarily.